1. Field of the Invention
The present invention relates to a nitride semiconductor laser device, and more particularly, it relates to a nitride semiconductor laser device, including growth surfaces of a-planes or m-planes, having an excellent high output characteristic and high reliability.
2. Description of the Background Art
A light-emitting device employing nitride-group III-V compound semiconductors is put into practice nowadays. Such a light-emitting device employs nitride semiconductor layers made of hexagonal nitride semiconductors, which are grown on the surface, formed by a c-plane, of a substrate made of sapphire or GaN. Therefore, the growth surface of each nitride semiconductor layer made of the hexagonal nitride semiconductor is the (0001) plane (c-plane).
The growth surface is parallel to the surface of the substrate, and the nitride semiconductor layer is mainly grown in the normal direction thereof. The nitride semiconductor grown on the c-plane has spontaneous polarization due to the hexagonal crystal system. When a heterojunction is formed with this hexagonal nitride semiconductor, strain results from lattice mismatching, to generate a piezoelectric field.
An internal field resulting from the spontaneous polarization and the piezoelectric field bends the band structure. In a quantum well employed for an active layer of a light-emitting device such as a light-emitting diode or a semiconductor laser device employing hexagonal nitride semiconductors, superposition of the wave functions of a conduction band and a valence band is reduced due to the band structure bent by the internal field, to reduce the luminous efficiency of the light-emitting device.
Particularly in the semiconductor laser device, the threshold current is disadvantageously increased due to the reduction in the luminous efficiency resulting from the internal field. Further, the strength of the piezoelectric field is increased as the strain is increased, and hence the luminous efficiency is remarkably reduced as the In composition is increased if the active layer is made of a nitride-group III-V compound semiconductor such as InGaN containing In. In this case, therefore, it is difficult to increase the wavelength of the laser beam emitted from the semiconductor laser device.
In order to solve such problems of the light-emitting device employing the nitride semiconductors grown on the c-planes, a light-emitting device employing nitride semiconductor layers made of hexagonal nitride semiconductors grown on (11-20) planes (a-planes) or (1-100) planes (m-planes) so that no internal field is generated is currently in the process of research and development (refer to Kuniyoshi Okamoto et al., “Continuous-Wave Operation of m-Plane InGaN Multiple Quantum Well Laser Diodes”, Japanese Journal of Applied Physics, Vol. 46, No. 9, 2007, pp. L187 to L189 (Non-Patent Document 1), for example).
However, while a technique of forming a protective film on a cavity facet in order to obtain an excellent high output characteristic and high reliability is disclosed (refer to Japanese Patent Laying-Open No. 2006-203162 (Patent Document 1), for example) as to a conventional nitride semiconductor laser device employing nitride semiconductor layers made of hexagonal nitride semiconductors grown on c-planes, no specific technique for obtaining an excellent high output characteristic and high reliability is examined as to a nitride semiconductor laser device including growth surface of a-planes or m-planes.